Embryonic Universe

February 6, 2012

Patterns of embryonic development provide important clues to the evolutionary relationships among animals. Animal cells differentiate into tissues during development that contrast with each other based on their function; these tissues are called layers . These patterns Diploblastic animals develop two embryonic cell layers and triploblastic animals develop three. In embryogenesis, the stage of embryo development, three distinct tissue layers form that will grow into organs, called the endoderm, the ectoderm, and the mesoderm. Ectoderm layers develop into nervous tissues, mesoderm layers develop into organs and muscles, and endoderm layers develop into the epithelial tissues that line the body cavity and surround organs.

Animals with radial symmetry, like cnidarians, produce two germ layers called the ectoderm and endoderm making them diploblastic. Some animals with bilateral symmetry produce a third middle layer between these two layers called the mesoderm making them triploblastic. These layers will develop into to all of an animal’s tissues and organs through the process called organogenesis that forms fully-mature organs and organ systems in animals. Diploblastic animals like cnidarians have only the ectoderm and endoderm tissue layers and thus because they lack the mesoderm, creatures like jellyfish don’t have organs. On Earth, triplobastic animals are divided into two categories based on their development. Differences in patterns of early development also characterize two major clades of triploblastic animals, the protostomes and deuterostomes.

A multicellular organism begins its development as an embryo. A series of embryonic stages will create an independent organism. In one of the earliest stages of embryonic development, a zygote will develop into a blastula. The blastula is a hollow sphere of cells formed during the early stages of embryonic development in animals. Gastrulation is another phase in the embryonic development of animals, during which the single-layered blastula is reorganized into a gastrula when the three distinct germ layers form. In the gastrulation of a diploblast some of the ectoderm cells migrate inward forming the endoderm layer. The archenteron is a primitive gut that forms during gastrulation in the developing blastula. A blastopore is an opening into the archenteron during the embryonic stages of an organism. The distinction between protostomes and deuterostomes is based on the direction in which the mouth, or stoma, develops in relation to the blastopore.

On Areios as on Earth, deuterostomes are distinguished by their embryonic development; in deuterostomes, the first opening called the blastopore becomes the anus, while in protostomes it becomes the mouth. In deuterostomes, the original dent becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth. For protostomes on Areios the dent formed the mouth while the anus was formed later, at the opening made by the other end of the gut. In protostome development, the first opening in development, the blastopore, becomes the animal’s mouth. In deuterostome development, the blastopore becomes the animal’s anus.

Cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. Cleavage ends with the formation of the blastula. Protostomes have what is known as spiral cleavage which is determinate, this meaning that the fate of the cells is determined as they are formed. Each cell produced by early embryonic cleavage does not have the capacity to develop into a complete organism. Deuterostomes have what is known as radial cleavage that is indeterminate; when the original cell in a deuterostome embryo divides, the two resulting cells can be separated, and each one can individually develop into a whole organism.

The genes that control the development of animals are an amazing testament and this is the single greatest piece of evidence to support the idea that all life on Earth shares a common origin. In upcoming posts, we will explore the identical genes common to all animals that regulate development and growth.

This graph outlines the defining characteristics of indeterminate deuterostomes and determinate protostomes.